1. Field of the Invention
The invention relates to a control apparatus and a control method for a direct injection spark ignition internal combustion engine.
2. Description of the Related Art
Tumble flows are created in a direct injection spark ignition internal combustion engine, and the tumble flows are moderately intensified using fuel injected at a timing near the intake stroke bottom dead center so that the tumble flows remain active until ignition. The intensified tumble flows make the movement of air-fuel mixtures at the time of ignition stronger, and this brings about a moderate increase in the combustion speed and thus provides a good condition for homogenous combustion. Among the technologies related to such fuel injection, for example, Japanese Patent Application Publication JP-A-2003-322022 recites a fuel injection control apparatus that intensifies a circulating flow in each engine cylinder using the fuel injected from a fuel injection valve. Further, as a technology for intensifying the intake flow in each cylinder during homogenous combustion, for example, Japanese Patent Application Publication JP-A-2005-180247 recites an intake control apparatus for a direct injection spark ignition internal combustion engine which intensifies the intake flow by controlling an intake flow control valve provided in an intake passage.
In recent years, there are known FFVs (Flexible Fuel Vehicles) that can run on mixed fuel containing gasoline and alcohol (will be simply referred to as “alcohol-mixed fuel” where appropriate) as well as on normal gasoline. Containing a different amount of carbon atoms from gasoline, alcohol-mixed fuel requires a stoichiometric air-fuel ratio lower than the stoichiometric air-fuel ratio for gasoline. More specifically, for example, when gasoline is used, the stoichiometric air-fuel ratio is about 14.5. When alcohol-mixed fuel is used, on the other hand, the stoichiometric air-fuel ratio may become 9 or so depending upon the alcohol concentration. Thus, when fuel having a property that requires a stoichiometric air-fuel ratio lower than the stoichiometric air-fuel ratio for gasoline, which may be fuel other than alcohol-mixed fuel, is used in a direct injection spark ignition internal combustion engine, the mass of the fuel per the air amount drawn into each engine cylinder during homogenous combustion becomes larger than it is when gasoline is used, and as a result, the airflow intensifying effect of the injected fuel becomes stronger. In this case, the combustion speed increases and therefore the cooling loss increases. As such, in FFVS, there is a possibility that the heat efficiency decreases due to an excessive increase in the combustion speed. Meanwhile, in the case where fuel having a property that requires a stoichiometric air-fuel ratio higher than the stoichiometric air-fuel ratio for gasoline, the mass of the fuel per the air amount drawn into each engine cylinder during homogenous combustion becomes smaller than it is when gasoline is used, and therefore there is a possibility that the airflow intensifying effect of the injected fuel becomes weaker and thus the intensity of tumble flow becomes insufficient.